The magnet is the most critical part of an MRI scanner and must have very good field homogeneity in a significant fraction of its internal volume. Dedicated scanners are described in patents such U.S. Pat. No. 6,191,584B1, US20080197845A1, US20100301861A1, US20110084695A1. Their magnetic structure includes: the magnetic material, the yoke and the magnetic poles. The homogeneity region of the magnet is contained between the poles and the part of the body under examination is inserted in this region. The presence of the yoke and of the iron pole has the following drawbacks:                Iron introduces magnetic losses, and this requires a greater amount of magnetic material to generate a given magnetic field;        It greatly increases size and weight of the magnet;        The shimming procedure becomes more complex and empirical due to the non-linearity of the iron magnetic susceptibility.        
A circular Halbach ring is a particular arrangement of permanent magnet blocks (see FIG. 2) which increases the magnetic field along one direction while canceling the orthogonal component of the magnetic field. This is achieved by rotating the direction of magnetization as a function of the azimuthal position of the blocks in the ring. One of the main advantages of a Halbach ring is that it does not require an iron yoke to confine the magnetic field. FIG. 2 shows different implementations: in FIG. 2a the direction of the magnetization changes continuously along the material; FIGS. 2b and 2c show implementations with a discrete number of blocks. In FIG. 2c the variation of the magnetization is obtained by rotating the blocks, on the contrary in FIG. 2b is the block magnetization inside the blocks to be rotated. The case of FIG. 2a implies a magnetic treatment of the entire ring which is technologically impractical particularly for a large magnetic device. The design of FIG. 2b requires that the blocks are magnetized, at the same value of the residual magnetization, along different directions. This is also difficult to obtain since the dimension of the blocks along the magnetization direction is different at different orientations. The main effect, in any case, is that a continuous distribution of the magnetization directions is replaced by a discrete distribution. The discretization has the effect of reducing the field uniformity along the radial direction, this reduction increases when the number of blocks in the ring is reduced. All the drawings of FIG. 2 are two-dimensional and involve an infinite length along the direction perpendicular to the plane. The MRI magnets have, obviously, a finite length and, in addition, the magnet length must be as short as possible to allow placing the part under examination in the center of the FOV. It is then necessary to find a different way to maximize the fraction of the magnet internal volume which has a suitable field homogeneity. In literature there are different designs of Halbach magnets aimed to increase the uniformity of the magnetic field. In particular H. Raich, P. Blumer “Design and Construction of a Dipolar Halbach Array with a Homogeneous Field from Identical Bar Magnets: NMR Mandhalas” Concepts in Magnetic Resonance Part B (Magnetic Resonance Engineering), Vol. 23B (1) 16-25 (2004) presents a 2D simulation using the finite element method of cubic blocks magnetized along an identical axis and placed at different orientations. The resulting magnet is formed by a stack of 8 identical rings and has a region of uniformity of about 20 mm. Bjørk R. et al. “Optimization and improvement of Halbach cylinder design”, J. Appl. Phys. 104, 013 910_2008, analyze the behavior of a hypothetical Halbach ring of cylindrical shape and describe the magnetic field shape as a function of the magnet height and of the inner and outer radius. Jachmann R. C. et al. “Multipole shimming of permanent magnets using harmonic corrector rings”, Rev. Sci. Instrum. 78, 035 115_2007, finally describe a 2D technique based on the correction of successive harmonics of the development of the field in series of powers. These papers, however, do not help to find a building modality which uses identical blocks, identical magnetization direction in a device with a large fraction of uniform magnetic field. Up to now, in spite of the fact that Halbach rings would represent a convenient way to build the magnets for small dedicated MRI, systems, they are not commonly used. Moreover the need to manufacture permanent magnetic blocks magnetized along different directions requires magnetization methods and equipment which are out of reach of most of magnetic materials producers. The U.S. Pat. Nos. 4,703,276, 5,148,138, 6,680,663, describe the use of blocks with different orientations of the magnetization direction. The patents US20090128272 and US20140111202 describe a similar principle, in the first the magnetic field is produced by four bars of magnetized material and the homogeneity region is a small fraction of the total volume. The second describes a portable device in which the size of the magnetized bars are very small and the resulting magnetic field is necessarily low. Both models also do not take into account the finite length of the bars causing a further reduction in the homogeneity region. An attempt to account for the finite length of the magnet is presented in patent WO 2007120057 in which the central blocks are placed at a distance calculated in such a way to cancel the second order coefficients of the field development in series power of (see WO 2007120057, claims 18, 21, 22, 23, 24, 25, 26) the result of this choice is that the uniformity of the region has linear dimensions ranging from one third to a tenth of the magnet height (see WO 2007120057 FIG. 3-8). A further element which tends to reduce the homogeneity of the field along the radial direction is that to obtain the required distribution of the magnetization directions using identical blocks they are rotated so as to present different surfaces in the radial direction including the edges of the blocks. This produces a non-uniform distribution of the potential which tends to reduce the homogeneity of the magnetic field inside.
In an exemplary embodiment, apparatus for diagnostic by magnetic resonance is configured to generate a uniform magnetic field in a field of view. The apparatus has a main longitudinal axis and includes two external Halbach ring of magnets positioned symmetrically with respect to a center of the longitudinal axis and a plurality of internal Halbach sets of magnets integrally coupled to a support structure formed by collars and rings, placed in pairs symmetrically with respect to the center of the longitudinal axis. The internal and external arrangements are positioned at different positions along the main longitudinal axis of the apparatus so that the two external arrangements are at the two end positions of the apparatus. The magnets of the external and internal arrangements are identical to each other and have a right prism shape with a N sides regular polygon base, whereby the right prism has a longitudinal axis and N side faces parallel to the longitudinal axis. Each magnet generates a magnetic field directed orthogonally to the center of one of its N side faces, and each of the external and internal arrangements of Halbach ring magnets includes a number P of elements arranged such that their longitudinal axes are positioned on a circumference centered on the main longitudinal axis of the apparatus, each one with an angular position and the longitudinal axes of two adjacent magnets have a mutual angular distance equal to 360°/N, and such that the magnetic fields of two adjacent magnets are oriented in directions forming an angle equal to 720°/N between themselves. The longitudinal axes of the magnets of each external arrangement are positioned on a circumference having a radius rest, and the longitudinal axes of the magnets of each internal arrangement are positioned on a circumference having a radius rint, where the radius rint is greater than the radius rest, wherein the longitudinal axis of each magnet of an internal arrangement is positioned at an angular position α equidistant between the angular positions of the two closest magnets of each of the two arrangements adjacent to that to which belongs the magnet under consideration, whereby the field of view has a shape of a sphere centered at the geometric center of the plurality of the internal arrangements of Halbach ring magnets. At least one of two external arrangements of Halbach ring magnets is provided with an opening configured to allow access to the field of view from outside.
In another exemplary embodiment, a method of homogenization of the magnetic field generated in the field of view by an apparatus for diagnostic by magnetic resonance is configured to generate a uniform magnetic field in a field of view. The apparatus has a main longitudinal axis and includes the two external Halbach rings of magnets of the described embodiments. The method includes the steps of (A) measuring the magnetic field generated by the two external arrangements of Halbach ring magnets and by the plurality of internal arrangements of Halbach ring magnets at a plurality of points of the field of view identified by the intersection of parallel and meridian curves; (B) calculating a magnetization to be applied to each of primary shim element in order to make the magnetic field homogeneous in the field of view; (C) magnetizing each primary shim element in accordance with the calculation of step B; (D) positioning in the apparatus the plurality of primary shim elements magnetized in step C; (E) measuring the magnetic field generated by the two external arrangements of Halbach ring magnets and by the plurality of internal arrangement of Halbach ring magnets, adjusted by the plurality of magnetized primary shim elements, at a plurality of points of the field of view; (F) calculating a magnetization to be applied to each secondary shim element so as to make homogeneous the magnetic field in the field of view; (G) magnetizing each secondary shim element in accordance with the calculation of step F; and (H) positioning the corresponding secondary shim element magnetized in step G in said at least one housing seat of each primary shim element.